1. Field of the Invention
This invention relates to an optical system for composing a linear image and to a recorder device utilizing the optical system for recording information on a photosensitive body.
2. Description of the Prior Art
In the prior art, an optical system for obtaining a micro linear image has been proposed, in which a cylindrical lens with negative reflectively is used to disperse a light beam in one direction and thereafter, the light beam is converged with a converging lens.
Such an optical system is shown in FIG. 1, in which the reference number 21 indicates a parallel light beam of a laser beam emitted, for example, from a laser beam source and expanded in width with a beam expander, usually taking a circular cross section. The parallel beam 1 is converted, with a mask 22 (shown in FIG. 2C) set in the light beam, having a rectangular opening 23, into a parallel light beam 24 with a rectangular cross section corresponding to said rectangular opening. The number 25 is a cylindrical lens with, for example, negative reflectivity, which diverges the light beam 24 only in the generatrix direction of the lens (the arrow mark S direction indicated by the dot line in FIG. 1b). As a result, the light beam incident upon a condenser lens 26 comprising convex lenses is a spherical wave diverging in the generatrix direction of the cylindrical lens 25, forming a parallel light beam in the direction vertical to the generatrix. When such a light beam is entered into the condenser lens 26, as shown in FIG. 1a, a parallel light beam component is first condensed on a focal plane 27 of the lens 26 and then diverged again, while a spherical wave component is not condensed on the focal plane 27 and as shown in FIG. 1b, it is condensed on another plane 28. This plane 28 is a composite focal plane of the cylindrical lens 25 and the condenser lens 26. Now, when the distribution of light on the focal planes 27 and 28, respectively is observed, as shown in FIG. 1d, an optical line extending in the generatrix direction of the clindrical lens 25 is obtained in the plane 27, while an optical line extending in the direction vertical to that generatrix is obtained on the plane 28. In addition, in FIG. 1d, the sign Y indicates the generatrix direction of the cylindrical lens and X the direction vertical to the generatrix, respectively.
In the optical system shown in FIG. 1, if the intensity distribution of a linear image, obtained on the focal plane 27, for example, is examined, the following fact will become obvious.
The intensity distribution of a linear image can be analytically obtained by utilizing Fresnel's diffractive integration. FIG. 2 shows the distribution in the vertical direction to the generatrix of the cylindrical lens 25 (that is, the width direction of a linear image) and the distribution in the generatrix (that is, the length direction of a linear image) (the signs X and Y in this drawing are the same as used in the previous description). As seen in FIG. 2a, while the width direction of a linear image has a smooth distribution represented by the function called Sinc-function, the length direction takes a distribution with complex intensity changes as shown in FIG. 1b. If a linear image with such latter intensity distribution is recorded on a photosensitive material such as a photo resist, the recording will have the contour of a concave-convex line as shown in FIG. 1c, because of the uneven exposure to the photo resist. That is, as shown in FIG. 1 the light beam has different focal planes in the directions vertical to each other. There has been employed another method for obtaining a linear image, in which a parallel light beam is irradiated to a converging lens through a shading plate having a slit-like opening to obtain a linear image on the focal plane of the converging lens. However, in this method, since a light beam is made in a slit-like shape due to the shading plate, the beam is almost intercepted with the shading plate permitting little to pass therethrough. Thus, the utility of the light beam is less efficient and the formation of a high energy linear image is difficult.